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- EXPERIMENT N O 1 DIODE CHARACTERISTICS...................................................... Table of Contents
- EXPERIMENT NO. 2 HALF WAVE RECTIFICATION
- EXPERIMENT N O 3 FULL WAVE RECTIFICATION ..................................................
- EXPERIMENT NO. 4 ZENER DIODE.............................................................................
- EXPERIMENT NO. 5 TRANSISTOR JUNCTION.........................................................
- EXPERIMENT NO. 6 COMMON BASE CONFIGURATION
- EXPERIMENT NO. 7 COMMON EMITTER CONFIGURATION
- EXPERIMENT NO. 8 COMMON COLLECTOR CONFIGURATION......................
- EXPERIMENT NO. 9 TRANSISTOR SPECIFICATION SHEET...............................
- EXPERIMENT NO. 10 JFET X-TICS AND COMMON SOURECE AMPLIFIER...
- EXPERIMENT NO. 11 JFET BIASING CIRCUIT........................................................
OBJECTIVE
When you have completed this exercise, you will be able to test a diode in a typical diode circuit by using a diode dc characteristic curve. You will verify your results with a multimeter.
DISCUSSION
Diode characteristic curves graph the current flow versus applied voltage. The right side of the graph shows the diode operating in the forward bias condition. The left portion of the graph illustrates the reverse bias condition. Voltage and current scales on the characteristic curve vary by region, ranging from milliamps to Pico amps or milli volts to volts. Forward voltages below the barrier voltage will not forward bias the diode. In silicon diodes, conduction begins when the forward voltage reaches between 0. and 0.7V. Germanium diodes require a forward voltage of about 0.3V. Once forward voltage produces conduction, small increases in voltage produce large current increases. This nearly constant voltage is called the forward voltage drop (VF). Reverse biased diodes will have very small current flows. The current flow is due to the presence of minority carriers and is referred to as leakage current. Breakdown voltage is the reverse voltage that causes large current flow through reverse biased diodes. The current can be large enough to damage the diode. In general, for calculations, the forward voltage drop of a silicon diode is 0.6V, and 0.3V for a germanium diode. Forward resistance is the ratio of the increase in voltage drop divided by the increase in forward current. Since forward resistance, or dynamic forward resistance, of a diode is very small, a resistor is included in the circuit to prevent damage to the diode. The highest current value that can pass through the diode without causing damage is referred to as maximum forward current (I F(max)). Ohmmeters cannot accurately measure the resistance of a diode junction therefore; diode junctions are tested by checking for continuity only. A forward biased junction will register a very low resistance indicating continuity. A reverse biased junction, or damaged diode, will indicate a very high resistance or an overload, indicating a lack of continuity. Included in this exercise are instructions for testing diodes with the following meters.
PROCEDURE
¾ Connect the circuit as shown in figure ¾ Check meter reading while selecting Ohms Scale of Your Multimeter
Exp # 01 DIODE CHARACTERISTICS
¾ Using Ohm’s law, find the current through R2. I R2 = _________________ ¾ Perform the following steps to determine the forward bias current (I D2 ) and voltage drop (VD) relationship for D2.
VA VR2 I D2 VD
0.75 V
5.00 V
¾ Sketch a graph between VD and I D2. ¾ Now verify above results VA I D2 = VR2 / 3.3 K VD - VA – V R 0.75 V 5.00 V
¾ Plot a new graph from the data calculated in above table between VD and I D2.
CNCLUSION
A diode DC characteristic curve describe the current flow for forward and reverse voltages. As the forward biasing voltage increases, the barrier voltage is overcome and the forward current increase rapidly with little change in diode Voltage drop. When the diode is reverse biased, initially only small leakage current flows until the reverse breakdown voltage e is reached. At the breakdown voltage, the reverse current increases rapidly and diode may be destroyed. Diode has a very low forward resistance and a very high reverse resistance.
OBJECTIVE
When you have completed this exercise, you will be able to demonstrate how a diode functions as a half-wave rectifier by using a typical half-wave rectifier circuit. You will verify your results with an oscilloscope and a multimeter.
DISCUSSION
Half-wave rectification converts an ac output to a pulsating dc output. The circuit consists of a diode and a load resistance. Either positive or negative pulsating dc output can be produced, depending on the way the diode is connected to the circuit. Rectification is the process of converting ac to dc. Half-wave rectification occurs when conduction is for only one half of every ac cycle. Dc output can be significantly lower than the ac input since the forward voltage drop of the diode must be reached before conduction occurs and voltage appears across the load. Half-wave rectification will be observed on an oscilloscope. Oscilloscope voltage measurements are peak-to-peak; therefore, the following conversion factor is used to convert the observed voltages to rms or average values. For average voltage V o(avg) = 0.318 x V o(pk) For rms voltage Vo(rms) = 0.707 x Vo(pk) These conversion factors are for the full half-cycle, so the calculated value will be less than the value measured with a multimeter. Variations in the pulsating dc output of a half-wave rectifier are referred to as ripple. Half-wave voltage rectifiers have ripple that is the same frequency as the input voltage frequency. The reverse recovery time (t RR ) of the diode can have an adverse affect on the output of a half-wave rectifier at frequencies larger than 1 kHz. Reverse recovery time causes an output voltage pulse in a direction opposite that of the normal half-wave pulse.
Exp # 02 HALF WAVE RECTIFICATION
CONCLUSSION
Half wave rectification is the process of converting an ac signal to pulsating dc signal during half of the ac input cycle. A diode and a load resistor form a half wave rectification. When the anode of the diode connects to the ac input signal, the output is a positive dc half wave pulsating signal. When the cathode of the diode connects to the ac input signal, the output is a negative dc half wave pulsating signal. The diode forward voltage drop reduces the output voltage of a half wave rectifier. Input frequencies greater than 1 khz can adversely affect the output due to the diode’s reverse recovery time. The pulsating dc output of a half wave rectifier is called ripple.
Full-wave rectifier - a diode configuration in which positive and negative alternations of an ac input signal are converted into a pulsating dc output signal.
OBJECTIVE
When you have completed this exercise, you will be able to demonstrate full-wave rectification by using a full-wave bridge rectifier circuit. You will verify your results with an oscilloscope and a multimeter.
DISCUSSION
A full-wave rectifier converts positive and negative alternations of an ac signal into a pulsating dc signal, as shown above.
A full-wave bridge rectifier is a circuit that performs full-wave rectification. The input to the bridge rectifier circuit is usually the secondary coil of a power transformer. The transformer isolates the bridge rectifier from the ac source and serves to step up (increase) or step down (decrease) the ac input to the bridge rectifier. Two types of circuits that utilize diodes for full-wave rectification are illustrated. One employs a center-tapped full-wave rectifier while the second uses a full-wave bridge rectifier. The full-wave bridge circuit is used in this exercise. Diode bridges contain four diodes, designated D1 through D4, configured so that two diodes conduct during each half-cycle of the input ac signal and produce a pulsating dc output. The pulsating dc output flows through the load resistance in one direction, independent of which ac cycle the current is derived. Two input terminals, usually labeled with a sine wave symbol, and two output terminals, labeled with positive and negative symbols, are present on the bridge rectifier. Diodes D1 and D3 are forward biased during the positive half-cycle of the ac input signal. Diodes D2 and D4 are forward biased during the negative half-cycle of the ac input signal. Each diode pair conducts for one half-cycle of the ac input signal, resulting in full-wave rectification.
Exp # 03 FULL WAVE BRIDGE RECTIFICATION
¾ Draw the wave form of voltage that you observe on oscilloscope of input and across D2. ¾ When D2 is conducting, measure VD2 with reference to ground. V(D2)= _________________
¾ When D2 is forward bias which another one Diode is also forward bias.
CONCLUSION
- A full wave diode bridge rectifier converts both phases of an ac input voltage to pulsating dc output voltage
- The output dc ripple frequency is two times the ac input frequency.
- The four diodes that form a full wave bridge rectifier are paired so that current flows in only two diodes at a time.
- The two conducting diodes steer the voltage so that the load current is always in the same direction
- The peak dc output voltage is less than the peak ac input voltage by the forward voltage drop of the two conducting diodes.
- The average voltage is 0.636 times the peak output voltage.
OBJECTIVE
When you have completed this exercise, you will be able to demonstrate how a filter significantly reduces the ripple of a pulsating dc output to a relatively smooth dc voltage by using a capacitive input filter circuit. You will verify your results with a multimeter and an oscilloscope.
DISCUSSION
Most electronic equipment requires a smooth dc voltage; therefore, filters are required after a rectifier to reduce ripple. Ripple present in the volt range can be reduced to the milli volt range. One example is the capacitive input filter which is implemented by placing an electrolytic capacitor across the bridge rectifier output and in parallel with the load resistor. Under no-load conditions the capacitor charges rapidly to the peak full-wave rectifier voltage output. Since there is no discharge path, the capacitor remains charged, maintaining the rectifier output when the rectifier input drops to zero. With a load present, the capacitor discharges through the load, maintaining a near-constant load voltage. Another output pulse recharges the capacitor before the capacitor can fully discharge. Discharge rate of a capacitor is longer than the charge time and depends on the RC (load Resistance times Capacitance) time constant. Discharge time, for constant frequencies, will affect the magnitude of the ripple. Increases in the capacitance and/or load resistance reduces ripple. Ripple is also reduced as frequency increases. Other types of filters are configured with inductors or combinations of resistors, capacitors, and inductors. Additional filter sections remove a portion of the rectifier ripple content. The circuit used in this exercise is not a regulated power supply. Parallel load resistance reduces the peak output voltage of the rectifier.
Exp # 03B POWER SUPPLY FILTERING
¾ Measure the ripple on oscilloscope when R3 is connected. And state the reason of these ripples. ¾ Using DMM measure the dc output voltage across C1, C2 and R. VO= _________________
CONCLUSION
- A capacitive input filter is electrolytic capacitor connected across a rectifier output.
- The capacitor charge very rapidly to the peak rectifier voltage.
- When the rectifier output drop to zero between pulses, the charged capacitor discharge and supplies the current to the load.
- Before the capacitor the voltage drop too low, another output pulses from the rectifier charged the capacitor to the peak voltage.
- The capacitor input filter discharge time constant is longer than the charge time constant.
OBJECTIVE
At the completion of this unit, you will be able to demonstrate wave shaping, zener diode operation and zener diode voltage regulation by using diode circuits.
FUNDAMENTALS
Diode circuits can be configured to output a waveform with a different shape than the input waveform. A limiter (or clipper) circuit removes all or part of the input alternation, as shown.
A clamper circuit shifts the positive or negative amplitude extreme of an input waveform to a different dc output voltage reference level, as shown. Limiter and clamper circuits take advantage of the diode forward bias voltage drop to set the clipping or clamping voltage level. A zener diode is designed to operate safely at the reverse breakdown voltage. The symbol for a zener diode is shown above.
Exp # 04A DIODE WAVE SHAPING
Series diode limiters, shown in this exercise are half-wave rectifiers. Parallel diode limiters are composed of a series resistor, diode and load resistance in parallel with the diode. When the diode is forward biased the output is limited to the diode forward voltage. The half-cycle input voltage appears at the output when the diode is reversed bias. A limiting circuit with a variable voltage bias at the diode is illustrated in this exercise. The point where limiting occurs is controlled by the amount of bias added to the circuit by a positive variable voltage supply. Clamping circuits shift the unwanted positive or negative portions of an input waveform to a different output dc voltage reference level. Clamping circuits are also referred to as restorers or baseline stabilizers. Clamping circuits with capacitors must have short charge time constants and long discharge time constants compared to the period of the input signal or the output waveform will be distorted.
PROCEDURE
¾ Connect the circuit as shown in figure; adjust the generator to 20 peak to peak voltages at 100 Hz sine wave signals.
¾ Connect the oscilloscope across R2 and observe the output waveform. ¾ Connect the Diode in a circuit as shown in figure below, adjust the initially to v1 at 0v dc.
¾ The circuit will work as limiter. ¾ Set the oscilloscope so that you can measure accurately peak voltage with respect to ground. VO(p)= _________________ ¾ The measure voltage in above step is forward voltage of diode. ¾ Now use the negative supply and repeat the above steps.
¾ Now connect the circuit as shown, when D1 and D2 are in circuit what kind of output will be.
¾ Adjust positive supply to 2.0 Vdc, Did the positive or negative alteration increased. ¾ Now adjust negative supply to -2.0 Vdc, Did the positive or negative alteration increased. ¾ Connect the circuit as shown in figure below, called Clamper Circuit.
¾ Adjust the generator to 10 peak to peak voltage and 1000 Hz Square wave. ¾ Observe the clamper circuit output. ¾ Measure the voltage level of the negative output peak. VO(p)= _________________ ¾ Adjust the positive variable supply to 3 Vdc using DMM. Using oscilloscope measure voltage level to which the positive output peak is clamped. VO(clamped)= _________________
¾ Connect the clamper circuit as shown in figure below, maintain the same generator setting and adjust the negative supply to 0 Vdc.
OBJECTIVE
When you have completed this exercise, you will be able to demonstrate the operation of a zener diode by using a dc characteristic curve. You will verify your results with a multimeter.
DISCUSSION
Reverse biased resistance of a diode is very large, so there is little current flow until breakdown voltage is reached (where reverse current increases rapidly). Breakdown voltage is also called avalanche or zener voltage and ranges from 3V to 200V. Zener diodes are PN junction diodes designed to operate at the avalanche voltage. The schematic symbol for a zener diode has a Z-shaped line at its cathode. A zener diode has a forward voltage the same as a conventional diode. Dc characteristic curve for a zener diode, in forward bias, is the same as a rectifier diode. In reverse bias direction, current flow is very small until zener voltage is reached. The knee of the curve is the zener region (breakdown point). Zener test current, specified by the manufacturer, is the current at which the zener voltage is within the tolerance range. The part of the curve where the current increases slowly with increases in the zener voltage is the soft region. At the stiff region of the curve, the current increases rapidly with very small increases in the zener voltage.
PROCEDURE
¾ Check Zener diode with DMM connecting positive terminal to anode and negative terminal to cathode. ¾ Check diode in reverse connection and record the readings of DMM. ¾ Connect the circuit as shown in figure below.
Exp # 04B THE ZENER DIODE
¾ Adjust the negative power supply to -2 Vdc at test point between R1 and R2. ¾ Measure the voltage across Zener diode. VD1 = _________________
¾ What your value indicate in the above step that diode is
- Forward bias
- Reverse bias ¾ Increase the negative variable supply to -2.5 Vdc at test point and measure diode voltage again. VD1 = _________________ ¾ Based on the above readings what would you conclude that the zener diode is functioning like a rectifier diode when it is forward bias. ¾ Connect the circuit as shown in figure below, Now positive variable supply is in the circuit.
¾ In the following steps you will determine the zener diode reverse current versus the reverse bias voltage by recording measurements and calculations in the table. ¾ Set the positive supply as said in the following table, Measure Diode voltage, Resistor voltage and compute Zener Current.
V A VD1 VR I Z 6.0 V 7.0 V 8.0 V 10.0 V
¾ Sketch a graph between I Z and VD. ¾ Compare your graph with standard zener diode VI characteristics curve of your book. ¾ I Z is about 0 milli A, when voltage across diode is less than 7 Vdc. State its reason in detail.
